A nerve cell can be excited in a number of different ways, but one direct method is to increase the electrical charge within the nerve, thus increasing the membrane potential inside the nerve with respect to the surrounding extracellular fluid. One class of devices that falls under the umbrella of Functional Electrical Stimulation (FES) realizes the excitation of the nerves by directly injecting charges into the nerves via electrodes which are either placed on the skin or in vivo next to the nerve group of interest. The electric fields necessary for the charge transfer are simply impressed via the wires of the electrodes.
FES is accomplished through a mechanism which involves a half-cell reaction. Electrons flow in wires and ions flow in the body. At the electro-electrolytic interface, a half-cell reaction occurs to accomplish the electron-ion interchange. Unless this half-cell reaction is maintained in the reversible regime, necrosis will result--partially because of the oxidation of the half-cell reaction and partially because of the chemical imbalance accompanied by it.
The advantage of FES is that the stimulation can usually be accomplished from extremely small electrodes with very modest current and voltage levels. The disadvantage however, is that it involves half-cell reactions. Most rehabilitation programs using FES place the electrodes directly on the skin. A conductive gel or buffering solution must be in place between the electrodes and the skin surface. Long term excitation of nerve or muscle tissue is often accompanied by skin irritation due to the current concentration at the electrode/skin interface. This problem is especially aggravated when larger excitation levels are required for more complete stimulation or recruitment of the nerve group.
By contrast, magnetic stimulation realizes the electric fields necessary for the charge transfer by induction. Rapidly changing magnetic fields induce electric fields in the biological tissue; when properly oriented, and when the proper magnitude is achieved, the magnetically induced electric field accomplishes the same result as realized by FES, that of transferring charge directly into the nerve to be excited. When the localized membrane potential inside the nerve rises with respect to its normal negative ambient level of approximately -90 millivolts (this level being sensitive to the type of nerve and local pH of the surrounding tissue), the nerve "fires."
The present invention is especially targeted at applications that are not suited for the use of implanted electrodes. The invention is designed for non-invasive external stimulation of selected nerve or nerve groups, particularly in certain applications. In these applications, which include incontinence and rehabilitation of muscle groups as well as potential weight loss treatment, the desired excitation levels using FES often fall outside of what might be considered comfortable limits. That is, the electrical current that ideally would be injected through the skin to excite the muscle groups of interest often leads to some skin irritation with time. The invention can also be used even in applications where this is not the case, as the use of gels and direct electrode/skin placement is inconvenient and is often resisted by the patient.
As opposed to FES, magnetic excitation has the attractive feature of not requiring electrode skin contact. Thus, stimulation can be achieved through the clothing that is being worn. This overcomes the objection of inconvenience and preserves the patient's dignity. Secondly, because there is no direct contact, stronger excitation levels can be realized without undue additional skin irritation. A contribution offered by the present invention is the ability to achieve higher levels of focusing of the magnetic field and thus stimulation within the patient. Commensurate with this greater level of focusing comes some flexibility in the number of possible applications that might be targeted. Also accompanying the focusing is a higher level of power efficiency. Typically, the devices being designed by the methods outlined in this invention reduce the magnetic reluctance path by a factor of two. This reluctance reduction translates into a diminution of the current by the same factor and a fourfold reduction in power loss.
Magnetic stimulation of neurons has been heavily investigated over the last decade. Almost all magnetic stimulation work has been done in vivo. The bulk of the magnetic stimulation work has been in the area of brain stimulation. Cohen has been a rather large contributor to this field of research (See e.g., T. Kujirai, M. Sato, J. Rothwell, and L. G. Cohen, "The Effects of Transcranial Magnetic Stimulation on Median Nerve Somatosensory Evoked Potentials", Journal of Clinical Neurophysiology and Electro Encephalography, Vol. 89, No. 4, 1993, pps. 227-234.) This work has been accompanied by various other research efforts including that of Davey, et al. (See, K. R. Davey, C. H. Cheng, C. M. Epstein "An Alloy--Core Electromagnet for Transcranial Brain Stimulation", Journal of Clinical Neurophysiology, Volume 6, Number 4, 1989, p.354); and that of Epstein, et al. (See, Charles Epstein, Daniel Schwartzberg, Kent Davey, and David Sudderth, "Localizing the Site of Magnetic Brain Stimulation in Humans", Neurology, Volume 40, April 1990, pps. 666-670). The bulk of all magnetic stimulation research attempts to fire nerves in the central nervous system.
The present invention differs in a number of respects from previous research efforts. First, the present invention has primary applicability to the peripheral nervous system, although it can be employed to stimulate nerves in the central nervous system as well. Second, and more importantly, the previous nerve stimulation work is dominated almost exclusively by air core coils of various shapes and sizes. The present invention is directed to the use of a magnetic core, more specifically a permeable core having a high field saturation, with the most preferred material being vanadium permendur. Among the air core stimulators are circles, ovals, figure eights, and D shaped coils. The coils are normally excited by a capacitive discharge into the winding of the core of these coils. This exponentially decaying field has a time constant typically in the neighborhood of 100 microseconds. Typical target values for the magnetic field peak happen to be near two Tesla. J. A. Cadwell is perhaps the leader among those who are now using and marketing these air core stimulators. Among his primary patents is U.S. Pat. No. 4,940,453 entitled "Method and Apparatus for Magnetically Stimulating Neurons" Jul. 10, 1990. There are a number of power supplies all of which operate on a basic capacitive type discharge into a number of air core coils which are sold with his units. Various shaped coils are being explored at this time. One such coil is a cap shaped device which fits over the motor cortex (K. Krus, L. Gugino, W. Levy, J. Cadwell, and B. Roth "The use of a cap shaped coil for transcranial stimulation of the motor cortex", Journal of Neurophysiology, Volume 10, Number 3, 1993, pages 353-362).
Some efforts are being given to various circuits used to fire these air core coils. H. Eton and R. Fisher offer one such alternative in their patent "Magnetic Nerve Stimulator" U.S. Pat. No. 5,066,272 Nov. 19, 1991. They suggest the use of two capacitors --one to capacitively discharge into the coil of interest, and a second to recover the charge from the inductive energy resident in the coil. The circuit used in the present invention accomplishes the same objective with a single capacitor.
Some stimulation research is being performed on the peripheral nervous system (See e.g., Paul Maccabee, V. Amassian, L. Eberle, and R. Cracco, "Magnetic Coil Stimulation of Straight and Bent Amphibian and Mammalian Peripheral Nerve in vitro: Locus of Excitation, "Journal of Physiology, Volume 460, January 1993, pages 201-219.) The bulk of Maccabee's work is however targeted for cranial excitation. The applications of the present invention focus on the peripheral nervous system although it can be used on the central nervous system, as well.